In addition to so-called spot measurements, in which a sample of a bodily fluid is taken from a user in a targeted fashion and examined with respect to the analyte concentration, continuous measurements are becoming increasingly common. Thus, in the recent past, continuous measuring of glucose in the interstitial space (also referred to as continuous monitoring, CM), for example, has been established as another important method for managing, monitoring and controlling a diabetes state. Continuous monitoring is often restricted to type I diabetics, that is to say diabetics who usually also carry an insulin pump. Continuous monitoring generally employs the use of directly implanted electrochemical sensors, which are often referred to as needle-type sensors (NTS). In the process, the active sensor region is applied directly to the measurement site, which is generally arranged in the interstitial tissue, and, for example, converts glucose into electrical charge by using an enzyme (e.g. glucose oxidase, GOD), which charge is related to the glucose concentration and can be used as a measurement variable. Examples of such transcutaneous measurement systems are described in U.S. Pat. No. 6,360,888 B1 and in US Pub. No. 2008/0242962 A1 both of which are incorporated herein by reference.
Hence, current continuous monitoring systems are generally transcutaneous systems. This means that the actual sensor is arranged under the skin of the user. However, an evaluation and control part of the system (also referred to as a patch) is generally situated outside of the body of the user, that is to say outside of the human or animal body. In the process, the sensor is generally applied using an insertion instrument such as that exemplified in U.S. Pat. No. 6,360,888 B1 which is incorporated herein by reference. Other types of insertion instruments are also known. A sensor is generally worn for approximately one week. After that, influences such as enzymes being used up and/or a sealing off in the body generally reduce the sensitivity of the sensor, or it is expected that the sensor fails. Increasing the duration of wear is an area of current research. However, this means that the sensor and, optionally, components directly connected to the former such as an insertion needle, should be designed as replaceable components. Accordingly, the sensor and optionally further replaceable components generally constitute a so-called disposable. By contrast, the evaluation and control part of the system is reused in most cases. Accordingly, this evaluation and control part is generally referred to as a so-called reusable.
Subdividing the transcutaneous sensor system like this into at least one disposable and at least one reusable in principle leads to the problem of requiring a detachable interface between the evaluation and control part (patch) and the sensor. This interface is generally subjected to being touched by the user to a greater or lesser extent, depending on the design of the system. This interface is subject to sterilization and disinfection insofar as the implantable sensor needs to be sterilized after production for reasons of biocompatibility and hygiene, and insofar as the reusable generally needs to be disinfected after use.
In principle, there are various means for sterilizing the sensor. However, since currently known sensors typically include sensor chemicals and have fluidic contact with the bodily fluid, for example the interstitial fluid, both chemical and thermal sterilization can generally be excluded. Traces of chemical sterilization, for example traces of ethylene oxide, could remain in the sensor chemicals, which, in the implanted state, could subsequently lead to ingress of the sterilization means into the body tissue. By contrast, thermal sterilization could destroy sensitive sensor chemicals, e.g. enzymes. Thus, in general, only radiation sterilization, for example using beta radiation, remains available as a method for sterilizing the sensor. However, radiation doses at the required strength often damage electronic components. Partial shielding of these components, as described in EP 1 178 841 B1, which is incorporated herein by reference, for example, requires great complexity in the production process.
Many currently pursued approaches for continuously monitoring analyte concentrations provide for a so-called bodymount to be stuck onto the skin of a user by means of a plaster. An example of such a system is described in US Pub. No. 2008/024962 A1 and EP 1 972 275 A1 both of which are incorporated herein by reference. The bodymount is likewise designed as a disposable. It contains a battery, an electronic storage medium (for example an EEPROM and/or a flash EPROM), one or more holding elements for the actual sensor, at least one plug connection and at least one hole in a base plate through which the sensor is inserted into the tissue of the user, for example by means of an insertion device. Charge-specific sensor data is stored in the storage medium because the sensor and the bodymount, as a disposable, generally form a packaging unit. Furthermore, operating modes can be stored in the storage medium. The sensor is inserted through the hole in the bodymount into the skin by means of an insertion aid. During the insertion, the insertion aid at the same time attaches the sensor, which is provided with a plug, to the bodymount such that the bodymount situated on the body is now provided with two fixed plug systems, namely a plug for attaching the sensor and a plug for attaching the reusable. The insertion aid is once again removed after the insertion. Liquid, for example blood or interstitial fluid, which emerges during the process and thereafter, can be removed by a swab. The so-called reusable is subsequently plugged on and affixed. The reusable has two sockets corresponding to the bodymount couplings. An electrical connection to the battery, the storage medium and the sensor electrodes is established via these sockets, and the measuring function of the continuous monitoring system is initiated on contact. Apart from the sensor electrodes, this system must generally be galvanically hermetically sealed during operation in order to avoid leakage currents and hence measurement errors.
There are a number of technical challenges in such systems. For example, potentiostatic electrochemical systems may generally only have very small leakage currents, for example at a reference electrode of the sensor, because currents in the picoampere range can chemically disintegrate the reference electrode. If such leakage currents disentgrate the reference electrode, the electrochemical system may fail prematurely. By contrast, leakage currents to or from the work electrode of the electrochemical sensor can lead to unidentifiable measurement errors. It is for this reason that the electrical lines and contacts in the system, with the exception of the individual sensor electrodes, should generally be completely isolated galvanically. This is generally only possible to a certain extent with much isolation complexity in the required current range. Environmentally-dependent interference penetrating the system, such as, for example, moisture, dust, salts or similar environmental influences can cause parasitic leakage currents. Hence, the system should be protected from such environmental influences where possible. However, the plug regions are necessarily accessible in the present concepts and there are often storage conditions that are counter to the requirement of high resistance (for example resistances of more than 109 ohms). Thus, in many cases, the high resistance in principle cannot be ensured over a relatively long period of use without targeted protective measures. Some gradual or imperceptible changes in the resistance conditions during ongoing intended operation can therefore only be determined with great metrological complexity in the case of the present signal magnitudes. However, if error detection were to be incorporated in the sensor interacting with the reusable within the scope of a fail-safe concept, this would lead to unjustified error messages in the case of over-sensitive discrimination, or would be ineffective in the case of insufficient discrimination.
Hence, the problems are generally concentrated on the side of the reusable. As discussed above, known concepts provide for multiple applications or even an application by a number of users. However, this means that the reusable is frequently handled by a user, and this cannot be controlled by the system. By contrast, the problem is less pronounced on the side of the sensor itself because said sensor is generally part of the disposable. This means that constructive precautions allow the sensor to be embodied such that it is only subjected to minimal access by the user. Adverse influences during production, storage and transport can be avoided by controlled processes. By way of example, U.S. Pat. No. 6,360,888 B1, which is incorporated herein by reference, has disclosed a sensor packaging system for storing and transporting a glucose sensor. Provision can also be made for an indicator in this packaging system that monitors the extent to which the sensor is subjected to high temperatures. WO 2006/133305 A2, which is incorporated herein by reference, also discloses protective packaging for an implantable biosensor. In principle, protective concepts are also known from different fields. For example, U.S. Pat. No. 4,801,271, which is incorporated herein by reference, discloses electrical connections that can briefly be closed using a dummy plug.
By way of example, current concepts provide for the sensor to be inserted into an insertion hollow needle after production and to be sterilized in an outer packaging. As a result, the sensor, including its contact area, is protected from being touched if used as intended. Moreover, moisture, dust and similar environmental influences are excluded during storage and transport by suitable packaging measures.
Thus, the described problems are predominantly on the side of the reusable. It needs to be ensured that the reusable is sufficiently protected from environmental influences after production, during transport, during temporary storage, and while the disposable components are being replaced. Although in principle it would also be possible to sidestep this problem by redefining a reusable as a disposable, this would not be an economical solution to the described problem in the case of large production numbers because the reusable generally contains extensive electronics.